To examine the surface reaction of the actinide oxides, the uranium oxide system was considered. The reactions of acetaldehyde are investigated over the surfaces of UO2, alpha-U3O8, and beta-UO3 by TPD and IR and under flow conditions. The reaction products showed a strong dependency on the O-to-U ratio in the general formula (UxOy). Reductive coupling is the dominant pathway on UO2, triggered by the capacity of the latter to accommodate large amounts of interstitial oxygen (UO2+x with x less than or equal to 0.25). Aldolization of two molecules of acetaldehyde to crotonaldehyde (CH(3)CHdouble bondCHCHO) prevails over alpha-U3O8. Over beta-UO3, the reaction products, at stoichiometric (TPD) as well as under flow conditions, are sensitive to surface coverage. At low coverage a cyclic compound, furan (C4H4O), is the dominant product, whereas both furan and crotonaldehyde molecules are formed at high coverage. Moreover, beta-UO3 could be easily transformed to either UO2 (deep reduction) or alpha-U3O8 (mild reduction) depending on the reaction conditions, and the dynamics between the three phases of the uranium oxides have implications with respect to reaction selectivity. In the absence of oxygen, acetaldehyde gave furan and crotonaldehyde but the selectivity to furan decreased sharply (within a few hours under flow conditions). XRD analyses of the catalyst after the reaction indicated a total transformation to UO2. The addition of oxygen increased the lifetime of the reaction of acetaldehyde to furan over P-UO3 but to the detriment of the selectivity. Although addition of oxygen did stop the deep reduction to UO2, considerable formation Of U3O8 was observed at the end of the reaction. IR analyses indicated that two modes of adsorption are noted for acetaldehyde depending on the oxide phase. Over UO3, acetaldehyde is adsorbed exclusively in eta(1) (O) configuration, while over UO2, the eta(2) (C, O) configuration is seen in addition. The latter configuration is reasonably linked to the reductive coupling of two acetaldehyde molecules to CH(3)CHdouble bondCHCH(3). (C) 2004 Elsevier Inc. All rights reserved.